verb: to make someone sitting in the back seat uncomfortably cold, by rolling down the front windows of the car. The perpetrator is usually a drunk fat-ass, and an inconsiderate boor, who is not interested in the comfort of fellow passengers. They can be easily identified by their robust neck meat. While the fatnecker is only interested in his own comfort, the person being fatnecked is usually too shy to ask the fatnecker to roll up his window, because the fatnecker is usually an older, burly, high school senior, and the victim, a skinny, waifish, freshman, who will have to earn the respect of his peers before dictating the climate of the vehicle..

"Man, that asshole fatnecked me all the way to Memphis last night. While he was in the front seat, sweating, and being loud and drunk, I was in the back seat, shivering, and too timid to ask him if he could roll up his window."

if you have a fat neck, it means you are proud, showing off, or bragging non-verbally. This term was coined when a friend of ours would get arrogant in a debate and tuck his chin in thus fattening his neck.